A water-stable and strongly luminescent self-assembled non-covalent lanthanide podate

The segmental ligand 2-(6-carboxypyridin-2-yl)-1,1-dimethyl 2-(5-methylpyridin-2-yl)-5,5-methylenebis(1H-benzimidazole) (L9) reacted with an equimolar mixture of LnIII (Ln = La or Eu) and ZnII in basic conditions to give selectively the self-assembled dinuclear non-covalent podates [LnZn(L9 – H)3]2+. Electrospray mass spectrometry and proton NMR spectroscopy show that [LnZn(L9–H)3]2+ adopt the expected head-to-head triple-helical structure with ZnII pseudo-octahedrally co-ordinated by the bidentate binding units of the three segmental ligands and LnIII occupying the remaining facial pseudo-tricapped trigonal prismatic site produced by the wrapped unsymmetrical tridentate units. Upon UV irradiation, solutions of [EuZn(L9 – H)3]2+ in acetonitrile or in water produce strong red luminescence. The Eu (5D0) lifetime and quantum yield indicate that EuIII is efficiently protected from external interactions for complex concentration in the range 10–4–10–8M and that no solvent molecule enters the first co-ordination sphere. Electrospray mass spectrometry combined with high-resolution emission spectroscopy confirm that the structure of the dinuclear triple-helical complex [EuZn(L9 – H)3]2+ is maintained at low concentration which strongly contrasts with the lipophilic analogous non-covalent lanthanide podates [EuZn(Li)3]5+ {i = 7 or 8; 2-[6-(organo)pyridin-2-yl]-1,1-dimethyl-2-(5-methylpyridin-2-yl)-5,5-methylenebis(1H-benzimidazole)} which are decomplexed in acetonitrile for concentrations below 10–5M. Detailed photophysical studies have established that [EuZn(L9 – H)3]2+ works as an efficient UV VIS light-converting device in the solid state and in water